1. Field of the Invention
This invention relates to a bottom lining for electrolytic cells and a process for its production. In particular, this invention relates to a bottom lining for aluminum electrolytic cells formed from steel pots for both primary production and secondary refining.
2. The Prior Art
Aluminum metal is produced by reducing metallurgical alumina by melt electrolysis at about 950.degree. C. to produce primary aluminum pig. High purity aluminum metal is produced from primary aluminum pig by refining electrolysis at about 800.degree. C. Fluoride-containing salt melts are used as the electrolyte in either process. In both processes, the electrolytic cell consists of a steel pot having dimensions of about 10 m in length and about 5m in width. The bottom surface of this pot is lined with carbon blocks that act as electrodes. A heat-insulating lining is present under the carbon. Generally, this lining consists of about 3 layers of dense chamotte bricks in the hotter zone and about 2 layers of highly porous diatomite bricks, vermiculite bricks, or calcium silicate plates in the colder zone underneath.
Bottom insulations produced by stamping metallurgical alumina powder or refractory masses of chamotte or olivine into the electrolysis pots are also known, and are sometimes employed instead of brick linings. Stamped insulating beds have an advantage over bricks and plates in that the monolithic, ramming mix has no joints to facilitate infiltration of the insulating bed by the electrolyte melt. Furthermore, less time and labor are required to produce a stamped bed. Moreover, as compared to bricks, which are solidly installed by mortar, a stamped insulating bed can be removed and replaced more easily and quickly after the useful life of the electrolytic cells has expired.
Molten and gaseous components of the electrolyte infiltrate the bottom insulation during operation of the cell and fill the open pores, leading to mineralogical transformation of the insulating material over time. This causes deterioration of the thermal insulating property of the bottom lining and limits the useful life of the electrolytic cell. In primary electrolysis, a used bottom lining made of a stamped insulating bed of metallurgical alumina can be completely reused as an aluminum oxide source in spite of its impregnation with electrolyte melt. In contrast, a consumed bottom lining based on chamotte brick or refractory compounds can only be partially reconditioned and reused.
As compared to chamotte bricks or a refractory ramming mix based on chamotte or olivine, which has a bulk density of about 2 to 2.5 g/cm.sup.3 and an open porosity of about 20%, a stamped lining bed made from metallurgical aluminum oxide powder, which has a stamping density of about 1.0 g/cm.sup.3 and an open porosity of about 70%, is a superior thermal insulator because of its higher porosity after an electrolytic cell has been put into operation. However, there is a greater structural change caused by infiltration of melt and thus deterioration of the thermal insulating property with an aluminum oxide stamping than with bricks because of its higher starting porosity.
Various processes are available for inhibiting infiltration of the bottom insulation. For example, chemical barrier layers based on SiO.sub.2 -containing melts formed by the reaction of electrolyte components with SiO.sub.2 -containing components of the bottom lining are used. In addition, physical infiltration barriers in the form of steel panels placed between the bottom carbons and the bottom insulation are known. The bottom insulation of an aluminum electrolytic cell is described in German Patent De 43 36 024, where the open pores of chamotte bricks are impregnated with tar to reduce the risk of infiltration.